System and method for cellular communications

ABSTRACT

A system for allocating subcarriers to subscribers, comprising a subcarriers allocation controller, connected to a subcarrier modulation unit in a transmitter and to a subcarrier demodulation unit in a receiver for setting a group of subcarriers to be used therein. The transmitter and the receiver are part of the transceiver and subcarriers allocation is made according to a Reed-Solomon code. In a multicarrier system, a method for allocating subcarriers to subscribers, comprising the steps of: A. keep a table of R-S codes for frequency group allocation to base stations; B. assign one set of subcarriers based on R-S codes to a base station; C. assign other sets of subcarriers based on R-S codes to other base stations in such a way that adjacent base stations have different R-S codes, to minimize the number of collision points therebetween.

FIELD OF THE INVENTION

The present invention relates to cellular and xDSL communicationsystems. More particularly, the invention relates to methods and systemsfor allocating subcarriers to subscribers in such a manner as to reducethe interference between users in the same cell and in adjacent cells orsectors.

BACKGROUND OF THE INVENTION

At present, a cellular system may use a multicarrier system. Suchsystems are known in the art, and may comply with the DVB-T standards inEurope, or MMDS (Multichannel Multipoint Distribution Service) in U.S.A.These systems use the Orthogonal Frequency Division Multiplexing (OFDM)method, wherein symbols are transmitted in parallel by modulating alarge number of orthogonal subcarriers.

A base station may be allocated, for example, 1700 out of 2000subcarriers. The subcarriers may span a given frequency range, whilebeing equidistantly located in the frequency domain.

The problem is how to allocate the various subcarriers to subscribers soas to minimize interference between users.

To illustrate such a problem: a base station is allocated, for example,a frequency range comprising 1600 subcarriers, divided into 64 groups of25 subcarriers each.

A first user is allocated a spectrum comprising a plurality ofsubcarriers, one in each of the 64 groups.

A second user is allocated a spectrum comprising a plurality ofsubcarriers, a different one in each of the 64 groups.

Preferably, all the subcarriers allocated to the second user aredifferent than those allocated to the first user.

Additional users are allocated other subcarriers.

One problem in the abovedetailed scheme is a possible collision betweenusers, when two users are allocated the same subcarrier. Such collisionsare undesirable, as they may generate errors in communication, which maypossibly increase the Bit Error Rate (BER) or slow down thecommunication channel.

Collisions may even occur if users are allocated different subcarriers.Thus, if there is a frequency deviation in the transmission of a firstuser and there are some subcarriers of that user which are close infrequency to some subcarriers of another user, then collisions mayoccur.

A collision occurs as the spectrum of that first user slides infrequency, to partially overlap with the spectrum of the second user.

Such frequency deviations may occur, for example, when one of the usersis in motion, because of the Doppler effect.

The need for the use of smart subcarrier allocation may come from thefrequency deviation that a user can suffer and its effect on otherusers. Allocating to users with all of their carriers allocated oneadjunct to the other can cause, in a frequency deviation scenario adestruction of the deviated user and the adjunct one.

As the number of colliding subcarriers increases, the undesireddisruption is more detrimental to communications.

Therefore, there is a problem of subcarriers allocation to users so asto reduce this undesirable effect.

The problem is further complicated in a multiuser cellular system, wherethere is a plurality of users communicating to a base station. Each useris allocated a plurality of subcarriers, some of which may possibly beclose in frequency to other user's subcarriers.

This is a complex, multidimensional problem that may be difficult tosolve.

The above description relates to communications between one base stationand users in that cell.

In real life, the problem is further aggravated because of interferencebetween adjacent cells. Thus, a base station in one cell may receiveinterfering signals (subcarriers) from users in a nearby cell.

It is an objective of the present invention to address the problems ofthe allocation of subcarriers in a multicarrier cellular system.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a system andmethod for allocating subcarriers to users in a multicarrier cellularcommunication system and xDSL systems. It includes the use of specialpermutation series for carrier allocation.

According to one aspect of the invention, the subcarriers in each cellare allocated according to a Reed-Solomon (R-S) sequence. Each user inthat cell is assigned a cyclic shifted version of a specific RS codesequence. This allocation decreases the number of subcarriers which areclose to each other, for the various users in that cell.

Two users may use a maximum of one case where two subcarriers areadjacent to each other.

Thus, a method and system for allocating subcarriers in a cell isdisclosed.

According to another aspect of the invention, users in adjacent cellsare allocated different R-S codes, or the same code where we are addingone to the index of each code.

This approach may be used to minimize interference in one base stationfrom users in another cell.

The use of R-S codes to frequency hopping in FH-CDMA is known in theart, as are known the low crosscorrelation properties of these codes.Until now, however, R-S codes have not been used for a group ofsubcarriers allocation in multicarriers systems.

According to yet another aspect of the invention, a distinction is madebetween the various users as to their interfering potential to a nearbycell, according to their distance from their own base station and theirsectorial location.

An improved subcarrier allocation method that takes these effects intoaccount is disclosed.

The present innovation may thus improve the reuse factor as well.

Thus, the invention relates to improvements in cellular and xDSLcommunication systems. These improvements include methods and systemsfor allocating subcarriers to subscribers in such a manner as to reducethe interference between users in the same cell and in adjacent cells orsectors.

Furthermore, the novel system and method will reduce the phase noise andinterference due to the Doppler effect to neighbor users who have beenallocated other subcarriers.

Further objects, advantages and other features of the present inventionwill become obvious to those skilled in the art upon reading thedisclosure set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example and with referenceto the accompanying drawings in which:

FIG. 1 illustrates a cellular wireless system.

FIG. 2 details a cellular transmitter/receiver including subcarriersallocation means.

FIG. 3 details a basic Reed-Solomon sequence of base 23 usable insubcarriers allocation.

FIG. 4 illustrates subcarrier allocation using Reed-Solomon sequences.

FIG. 5 details a method for subcarriers allocation to users in acellular system.

FIG. 6 details another method for subcarriers allocation to users in acellular system.

FIG. 7 further illustrates permutations used in subcarriers allocation,based on a Reed-Solomon (R-S) code.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will now be described byway of example and with reference to the accompanying drawings.

FIG. 1 illustrates a cellular wireless system with a plurality ofwireless cells 11, 12 and 13 as illustrated, and a corresponding basestation 111, 121, 131 in each cell.

In each cell there may be, at any given moment, a plurality ofsubscribers like the mobile subscribers 122, 123 and 124 in cell 12.

According to the present invention, there is made a distinction betweensubscribers close to a base station and subscribers closer to theperiphery of a cell. The illustrated inner circle 126 separates betweenclose subscribers like subscriber 124 which is located within circle126, and more distant subscribers like 122 and 123 which are locatedwithin the outer ring 127.

The close subscribers transmit at a lower power, since they are close tothe base station and need not transmit a high power. Therefore, closesubscribers will interfere less with another base station, than distantsubscribers. For example, subscriber 122 which is more distant from base121, will interfere more with base 111 than the closely locatedsubscriber 124.

Accordingly, a distant subscriber like subscribers 122 and 123 may beassigned different codes than close subscribers, to achieve a betterseparation from the codes used in adjacent cells.

A method for achieving better separation between users in a cellularsystem, using this effect, is further detailed with reference to FIGS. 5and 6.

The above system and method may achieve less interference between usersin the cellular system, thus increasing the number of users who may beconcurrently serviced in the system.

Where directional information is available in the base station, it ispossible to further reduce interference between users and base stations.

The novel system and method uses a novel approach of directional codeassignment, which is based on the discovery that subscribers indifferent sectors cause a different amount of interference to adjacentbase stations.

For example, there is definitely a different amount of interference tobase station 111 from subscribers in sectors 138 and 139 in cell 13. Amobile subscriber 132 in sector 138 will cause a strong interference,whereas a mobile subscriber 133 will cause a weaker interference.

The reason is wave propagation phenomena, wherein the intensity of theradio waves decreases as the fourth power of range, for transmitters andreceivers close to ground.

Therefore, base station 131 may allocate different codes to subscribersin cell 13, taking into account possible interference to adjacent cells.Subscriber 132 will be assigned a code that is more farther away fromthat in base station 111, to counter the stronger interference caused bythat subscriber, because of its closer distance.

Subscriber 133, however, may be assigned a code that is closer to thecodes in base 111, since the much larger distance (about 2 to 3 timeslarger) will cause a decrease of 14 to 27 times in the power ofinterference to that base station.

Base station 131 is illustrated as having six sectors. A differentnumber of sectors may be used. In any case, there is informationavailable in the base station regarding the direction of eachsubscriber.

According to the invention, this information is used to allocatesubcarriers to each subscriber, so as to minimize the interferencebetween the subscribers and the base stations in the cellular system.

Such a smart subcarrier allocation will decrease interference in thesystem and will thus allow more users to operate in the allocatedfrequency band.

FIG. 2 details a cellular transmitter/receiver including subcarriersallocation means. The system may be used in a cellular multicarriersystem, wherein subcarrier groups are allocated to eachtransmitter/receiver according to the methods detailed in the presentdisclosure.

The transmitter may include a transmit data in (serial) 211 beingapplied to a serial to parallel converter 212, which convert the data toparallel words. A subcarrier modulation unit 213 modulates thesubcarriers allocated to the transmitter responsive to the data in 211.

A subcarriers allocation controller 22 allocates the subcarriers to beused in unit 213, out of the set of allocated subcarriers for that basestation.

The Inverse Fast Fourier Transform (IFFT) unit 214 transforms themodulated subcarriers into the time domain, for transmission.

The IFFT unit is just one embodiment of a multicarrier modulation unit;other implementations of the unit are possible as well.

Since the time samples are concurrently being output out of unit 214, aparallel to serial converter 215 is used to transform the samples toserial form for transmission. The resulting signal is output as thetransmit data out (serial) 216.

The receiver may include a receive data in (serial) 231 being applied toa serial to parallel converter 232, for the subsequent signalprocessing.

A Fast Fourier Transform (FFT) unit 233 converts the received signalsinto the frequency domain. A subcarrier demodulation unit 234 usesinformation regarding subcarriers in the received signal received fromthe subcarriers allocation controller 22 to demodulate the receivedsignal.

A parallel to serial converter 235 reconstructs the received signal inthe time domain, this being the receive data out (serial) 236.

FIG. 3 details a basic Reed-Solomon (R-S) code 23 based, usable insubcarriers allocation.

These codes have the maximal distance between shifted versions thereof,and accordingly are advantageously used in the present invention in amulticarrier cellular system to minimize collisions between subcarriers.There are 23 cyclic rotations, from user to user.

Each row in the table corresponds to one cell or sector. Thus, forexample, the first row may correspond to Cell 233/sector #1, the secondrow may correspond to Cell 233/sector #2, etc.

FIG. 4 illustrates an example of subcarrier allocation usingReed-Solomon codes. The method may be used to allocate subcarriers in amultcarrier system such as OFDM, filtered multitone (FMT), discretemultitone (DMT), etc.

Assuming a spectrum of available subcarriers 31 comprising subcarriersnumber 0, 1, 2, 3, 4, . . . N, the subcarrier allocation first user 32may be as illustrated. The value points to the allocated carrier withineach group of subcarriers and its index is the sub-group numbering.

The subcarrier allocation for a second user 33 may be as illustrated,that is a shifted version of that for the first user 32.

It can be seen that adjacent subcarriers are separated by more than onestep difference, except between subcarriers 16 and 17 in zone 323.

Similarly, there is the subcarrier allocation for a third user 34, asillustrated. Again, good separation between subcarriers is achieved,except one step difference in zone 334.

The Reed-Solomon series works on a prime number and generates all thenumbers to this prime in a special permutation. The series has thespecial property that every cyclic permutation of the series gives thesame result, and between any to cyclic permutation only 2 numbers areadjunct.

Therefore, if users are allocated carriers with groups of prime numbers,using the Reed-Solomon series for their carrier allocation within eachcarrier group, two users will have only one adjunct carrier to otherusers. If a user suffers from a frequency deviation, then only onesubcarrier of all users is damaged, and an error correction code (ECC)scheme can handle it.

The next series is a Reed-Solomon series from the prime number of 23:

-   0,5,2,10,4,20,8,17,16,11,9,22,18,21,13,19,3,15,6,7,12,14,1

After two cyclic permutations we get:

-   2,10,4,20,8,17,16,11,9,22,18,21,13,19,3,15,6,7,12,14,1,0,5

For this cyclic permutation we see that for a group were the 22 carrieris allocated to one user another users will be allocated the 21 carrier,and only those two will be adjunct for all the carrier allocations.

The carrier allocations are done with groups of carriers containing theamount of prime number carriers. This scheme of allocation can mitigatethe effect of one frequency deviated user (it may also be mobile), andtherefore we may consider less modes of operation, and a simplifiedsystem.

Another advantage for this carrier allocation is derived from thefrequency reuse that could be achieved. By using a different derivedbasic allocation we can allocate carriers in a different cell that usesthe same frequency in such a way that the cross correlation of theallocation will be minimal. Therefore average the interference from thatcell, this technique will enable to better deal with Co-ChannelInterference (CCI) and help in the frequency reuse.

Subcarriers Allocation Method

An example of subcarriers allocation using RS codes may include:

A. all usable subcarriers are divided into 23 carrier groups, calledbasic groups. According to implementation, a different number of basicgroups may be used.

B. each group contains 23 carriers.

C. a first user is assigned carriers in the above groups, as follows

-   in group No. 1 2 3 4 5 6 7 8 9 10 11 12-   Subcarrier No. 0 5 2 10 4 20 8 17 16 11 9 22

D. a second user is assigned carriers in the above groups, as follows

-   in group No. 1 2 3 4 5 6 7 8 9 10 11 12-   Subcarrier No. 5 2 10 4 20 8 17 16 11 9 22 18

E. a second user is assigned carriers in the above groups, as follows

-   in group No. 1 2 3 4 5 6 7 8 9 10 11 12-   Subcarrier No. 2 10 4 20 8 17 16 11 9 22 18 21

F. subsequent users will be assigned subcarriers according to the samemethod as illustrated above.

One will recognize that the first user received one permutation of RScode, spread over the various groups of subcarriers.

The second user received the same sequence, shifted one location.

The third user received the sequence shifted two places, etc.

The number of groups and the number of subcarriers in each group mayvary, according to the number of subcarriers available.

For example, if more subcarriers are available, they may be furtherdivided into three super-groups. Such a scheme will accomodate 3*23*23subcarriers.

Additional details are presented with reference to FIG. 7.

The above method may be used with the various systems and methodsdetailed in the present disclosure.

The subcarriers allocation is dynamic, as users enter or leave cells anda controller allocates subcarriers and cancels previous allocations thatare no longer in use.

FIG. 5 details a method for subcarriers allocation in a cellular system,usable in a cellular system by the network manager, to allocatesubcarriers to the various base stations, as follows:

Task 400: keeps a table of R-S codes for frequency group allocation tobase stations

Task 405: assigns one set of subcarriers based on R-S codes to a basestation

Task 410: assigns other sets of subcarriers based on R-S codes to otherbase stations in such a way that adjacent base stations have differentR-S codes, to minimize the number of collision points therebetween.

In an OFDM system having N subcarriers available, numbered 0 to N-1, afirst base station will be assigned subcarriers Fa, Fb, Fc . . . wherea, b, c . . . . are members of an R-S code.

For example:

-   0, 5, 2, 10, 4, 20, 8, 17, 16, 11, 9, 22, 18, 21, 13, 19, 3, 15, 6,    where the above numbers refer to the abovedetailed subcarriers 0 to    N−1.

An adjacent base station will be assigned a different R-S code, forexample:

-   8, 5, 13, 7, 0, 11, 20, 19, 14, 12, 2, 21, 1

Task 415: where a base station has sectored coverage, a plurality ofcodes is assigned to that station for use with the various sectors. Endof method.

FIG. 6 details another method for subcarriers allocation to users in abase station in a cellular system, as follows:

Task 440: base station keeps a table of available codes, wherein part ofthe codes are tagged “free” whereas the others are “in use”

Task 445: when a new subscriber gains access through a base station, heis assigned one of the codes for that cell.

For example, the code may include the subcarriers numbered:

-   -   0, 5, 2, 10, 4, 20, 8, 17, 16, 11, 9, 22, 18, 21, 13, 19, 3, 15,        6, . . .

That code is tagged “in use” or “occupied” in the base station, toprevent its double assignment. Thus, each subscriber in a cell willreceive a unique code from the base station.

If several sectors are used, the same code may be assigned to severalsubscribers, each in a different sector.

Task 450: when a subscriber leaves the cell, his R-S code is tagged as“free”

Task 455: a new subscriber is assigned a shifted version of the code

Task 460: different codes are allocated in various sectors, and takinginto account the code of the nearby cell.

Task 465: optional: several codes are allocated to each station fornear/far subscribers. Separating far/near subscribers using differentcodes achieves better separation from adjacent cells. End of method.

FIG. 7 further illustrates the permutations used in subcarriersallocation. The carriers are based on a Reed-Solomon (R-S) code 23based, as detailed in FIG. 3 above.

The usable carriers are divided into 23 carrier groups named “basicgroups”. Each group contains 23 basic groups.

There are three main groups, as illustrated.

These codes have the maximal distance between shifted versions thereof,and accordingly are advantageously used in the present invention in amulticarrier cellular system to minimize collisions between subcarriers.There are 23 cyclic rotations, from user to user.

The three main groups are indicated in white, hashed and grayrectangles, respectively. Within each group, Reed-Solomon (R-S) codesare used.

It will be recognized that the foregoing is but one example of anapparatus and method within the scope of the present invention and thatvarious modifications will occur to those skilled in the art uponreading the disclosure set forth hereinbefore.

1. A system for allocating subcarriers to subscribers, comprising: A. aserial to parallel converter for the serial transmit data; B. asubcarrier modulation unit connected to said converter; C. a subcarriersallocation controller connected to the modulation unit; D. amulticarrier modulation unit; and E. a parallel to serial converter,which generates the transmit data out (serial), wherein the subcarriersallocation controller allocates subcarriers using a Reed-Solomon (R-S)codes scheme, and wherein the subcarriers allocation controllerallocates subcarriers using a shifted version of a Reed-Solomon (R-S)code that are separated by more than one step difference.
 2. The systemfor allocating subcarriers to subscribers according to claim 1, whereinthe system is a cellular communication system.
 3. The system forallocating subcarriers to subscribers according to claim 1, wherein thesystem is a xDSL communication system.
 4. The system for allocatingsubcarriers to subscribers according to claim 1, wherein a group of 22carriers is allocated to one user, another user will be allocated acycled version of the group.
 5. In a multicarrier system, a method forallocating subcarriers to subscribers, comprising the steps of: A. keepa table based of Reed-Solomom (R-S) codes for frequency group allocationto base stations; B. assign one set of subcarriers based on allocationusing the R-S codes assigned to a base station; C. assign other sets ofsubcarriers based on R-S codes to other base stations in such a way thatadjacent base stations have different R-S codes, to minimize the numberof collision points there between, further including the steps of: C1.where a base station has sectored coverage, a plurality of R-S codes areassigned to that station for use with the various sectors; C2. basestation keeps a table of available R-S codes, wherein part of the R—Scodes are tagged “free” whereas the others are “in use”; C3. when a newsubscriber gains access through a base station, the subscriber isassigned one or more of the R-S codes for that cell according hisassigned bandwidth (BW); C4. when a subscriber leaves the cell or whenhe does not have assigned BW, his R-S code is tagged as “free”; C5. anew subscriber is assigned a shifted version of the R-S code; C6.different R-S codes are allocated in various sectors, and taking intoaccount the R-S code of the nearby cell.
 6. The method for allocatingsubcarriers to subscribers according to claim 5, further including thestep of: D. allocating several codes to each station for near/farsubscribers, and separating far/near subscribers using different codes.7. The method for allocating subcarriers to subscribers according toclaim 5, further including the step of: E. allocating several codes toeach of several adjacent users within one cell, to decrease theinterference resulting from Doppler, phase noise or collisions withother subscribers.
 8. The method for allocating subcarriers tosubscribers according to claim 5, wherein the code includes thesubcarriers numbered: 0, 5, 2, 10, 4, 20, 8, 17, 16, 11, 9, 22, 18, 21,13, 19, 3, 15,
 6. 9. The method for allocating subcarriers tosubscribers according to claim 5, wherein the multicarrier system is acellular communication system.
 10. The method for allocating subcarriersto subscribers according to claim 5, wherein the multicarrier system isa xDSL communication system.
 11. The method for allocating subcarriersto subscribers according to claim 5, wherein a group of 22 carriers isallocated to one user, another user will be allocated a cycled versionof the group.